The present invention relates to a blood pressure measurement device and a method of calculating an index of the degree of arteriosclerosis using the device, and in particular to a blood pressure measurement device measuring blood pressure data useful in determining an index of arteriosclerosis, and a method of calculating an index of the degree of arteriosclerosis using said device.
Previously, devices determined the degree of arteriosclerosis by finding the speed of propagation of pulse waves emitted from the heart (PWV: pulse wave velocity). Because the speed of pulse wave propagation becomes faster as arteriosclerosis advances, the PWV is a useful indicator for determining the degree of arteriosclerosis and has continued to be generally used in medical facilities etc. as the standard indicator for determining the degree of arteriosclerosis up to the present time. PWV measurement devices measure pulse waves by affixing a cuff etc. to at least two locations such as the upper arm or lower extremities etc., and thereby are able to calculate the difference in time between appearance of each pulse wave (ejection wave, reflected wave) from the length etc. of arteries at the two points where the cuffs etc. are attached to measure the pulse wave. This time difference is used as the Tr (Traveling time to reflected wave) and is another indicator of the degree of arteriosclerosis.
However, the equipment required to perform the aforementioned PWV measurement is expensive. Furthermore, because of the requirement to attach the cuffs to at least two locations such as the upper arm or lower extremities etc. in order to measure the pulse wave, it is difficult to measure pulse wave propagation velocity PWV easily at home. Accordingly, technologies have been proposed whereby the degree of arteriosclerosis is determined only from the pulse wave at the upper arm or carotid artery.
Technology for determining the degree of arteriosclerosis from the pulse wave in the upper arm only, such as in Patent Laid-open 2004-113593 (“Patent Reference 1”) (the disclosure of which is incorporated herein by reference), discloses an evaluation device providing a cuff for measuring the pulse wave and a pressure cuff compressing the peripheral end. Using this device, it is possible to compress the peripheral end while measuring the pulse wave at the heart end. By this means, the ejection wave ejected from the heart can be separated from the reflected wave from the iliac artery branch and various parts of the artery. Thus, it is possible to determine the degree of arteriosclerosis by calculation of the time difference and ratio of strength of the peaks of the advancing wave component and reflected wave component(s).
In order to accurately determine the degree of arteriosclerosis by means of the technology disclosed in Patent Reference 1, it is necessary to accurately detect the point of origin of a reflected wave from the pulse wave. A method for this purpose, such as in Patent Publication 2009-517140 (the disclosure of which is incorporated herein by reference), is disclosed as a method of separating the ejection wave and reflected wave using the estimated values of the blood pressure waveform and blood flow volume waveform of the aorta. FIG. 16(A) and FIG. 16(B) are drawings for the purpose of explaining the method of Patent Reference 2, wherein a ejection wave (the advancing wave in the drawing) and reflected wave are separated from a blood pressure wave, which is a composite wave of an ejection wave and reflected wave as shown in FIG. 16(A).
In the method according to Patent Reference 2, a blood pressure waveform estimated by the transfer function method, from the blood pressure wave measured at a peripheral artery in the upper body (such as the radial artery or brachial artery etc.), or a blood pressure waveform measured from the carotid artery, is used to approximate the value of the blood pressure wave of the artery. The aforementioned transfer function method is disclosed in U.S. Pat. No. 5,265,011. As for the blood flow volume wave, as stated in Non-patent Reference 1 (B. E. Westerhof et al., Quantification of wave reflection in the human aorta from pressure alone: a proof of principle. Hypertension 2006; 48; 595-601) (the disclosure of which is incorporated herein by reference), a triangular waveform is used, taking from the rise of the blood pressure waveform to the incisural notch as the base, and the peak or heart contraction as the apex. In the method according to Patent Reference 2, the ejection wave and reflected wave are thus separated, and the mutual relationship thereof is calculated, and the time of highest correlation is detected as the time difference between appearance of the ejection wave and reflected wave.
However, in the aforementioned mutual relation method, it is possible to detect the time difference between the appearance of the two waveforms with good accuracy from mutual correlation if the two waveforms are respectively similar to each other. However, if the shapes of the waveforms are different, the margin of error in detection of the time difference between the appearance is increased. The blood pressure wave changes in form after the ejection wave from the heart propagates through the aorta. Also, the manner of this change in form varies according to the degree of arteriosclerosis etc. and the state of the patient. Thus, there are instances in which the time difference between the appearance of the ejection wave and the appearance of the reflected wave cannot be detected accurately from mutual correlation.
FIG. 17 shows the relationship between Tr calculated from pulse wave propagation time between two points, measured by a PWV measurement device according to prior art (“PWV Tr”), and Tr derived using prior art from actual measurement of the blood pressure wave in the carotid artery in approximately 200 individuals. The PWV Tr value calculated from pulse wave propagation velocity measured between the two points of the heart and femoral aorta using a PWV measurement device and the propagation distance of said two points is considered the most accurate Tr value that can be measured using a non-invasive measurement device at the present time. In comparison, the aforementioned Tr value derived from the blood pressure waveform in the carotid artery is obtained by detection of a time difference, between a ejection wave and a reflected wave separated using a blood flow waveform in the shape of a triangular wave and a blood pressure waveform, by means of the aforementioned mutual correlation method. From the results in FIG. 17, it is clear that in many subjects, the value of Tr derived from the aforementioned blood pressure wave in the carotid artery is calculated to be longer than the Tr value obtained using a PWV measurement device. This result is considered to indicate that a clear margin of error is present in a time difference between the appearance of the ejection wave and the reflected wave detected by means of a mutual correlation method from the pulse waveform in the carotid artery.
Also, as a method of determining the rise point of the pulse wave (reflected wave), a method is known whereby a given percentage of the pulse wave amplitude (for example 10% or 20%) is set as a threshold value, and the point where said threshold value is reached is estimated to be the rise point of the reflected wave. FIGS. 18(A) and (B) describe a method of estimating the rise point of the reflected wave using a threshold value. The ejection wave and reflected wave (FIG. 18(A)) from measured blood pressure waveforms in measured subjects are separated using the method of mutual correlation, and then the maximum amplitude of the reflected wave is enlarged until it is the same as the maximum amplitude of the ejection wave (FIG. 18(B)). Assuming that the threshold value is set at 20%, the X-axis coordinate of the point at which the amplitude of the reflected wave reaches the maximum amplitude of the reflected wave (“1” in FIG. 18(B)) multiplied by the 20% threshold value (“0.2*”) is estimated to be the rise point of the reflected wave. Tr is calculated by estimating the time difference between the rise point of the ejection wave and the rise point of the reflected wave (FIG. 18(B)). However, as described above, in cases where the shapes of the blood pressure waveforms are not similar to each other and are large, then even the use of the threshold value ratio as described above will not in some cases accurately detect the time difference between the appearance of the ejection wave and the reflected wave.
Accordingly, one or more embodiments of the present invention provide a blood pressure data measurement device capable of accurately calculating a useful index for determining the degree of arteriosclerosis by accurately detecting the time difference in the appearance of the ejection wave and the reflected wave from the blood pressure waveform, and a method of calculating an index for the degree of arteriosclerosis by means of said device.